1. Technical Field
The present invention relates to a transmit power control method in a wireless communication system, and a base station and a mobile station used for the transmit power control.
2. Background Art
Terminals (wireless communication terminals) for wireless communication, such as mobile phones, PHS, PDA, pagers or wireless LAN connection devices have become popular, and increased expansion of service areas for the wireless communication and more diversification of services are expected.
There are various types of wireless communication system based on wireless communication technology, such as the TDMA (Time Division Multiple Access) system and the CDMA (Code Division Multiple Access) system. Here, the CDMA system will be described as a representative thereof.
In the wireless communication system based on CDMA, communication is conducted in the following manner. First, an apparatus on the sending side spreads a transmission signal by using a spreading code assigned to a user on the sending side and then transmits the spread signal. An apparatus on the receiving side receives the transmission signal and despreads the received signal in order to restore the original signal by using the same spreading code as the spreading code assigned to the sending side user.
In the CDMA system, since it is possible to distinguish each communication entity of a plurality of communication entities by using different spreading codes that are orthogonal with each other, a plurality of communication entities can use a radio signal of the same frequency.
However, it is difficult to completely preserve the orthogonality of all of the spreading codes used, and in practice the respective despread codes are not completely orthogonal with each other and have components that correlate with other codes. These correlating components constitute interference components at the time of communication, causing deterioration in communication quality. Accordingly, as the number of communication entities increases, the interference also increases.
Further, in wireless communication systems in general, information transmission is conducted through a base station and not by a direct connection between mobile stations (corresponding to wireless communication terminals). Hence, a situation arises in which a signal from a mobile station that is close to a base station imparts interference to a faint signal of a mobile station located at a distance from the base station. In the CDMA system, the transmission power of mobile stations is controlled in order to prevent deterioration of communication quality due to this problem (known as the “far-near problem”).
3GPP TS 25.214 V3.12.0, p11-21 (Apr. 4, 2003) shows an uplink power control in W-CDMA (Wideband-CDMA: one of IMT-2000) as an example. A base station controls transmission power of each mobile station so that the Eb/IO (desired reception wave energy per bit (Eb) to noise and interference wave power (IO) ratio) or SIR (Signal to Interference ratio) for a communication link established between the respective mobile stations is high enough to secure a predetermined minimum communication quality. More specifically, a downlink signal transmitted from the base station to a mobile station includes TPC (Transmit Power Control) information for instructing the mobile station to increase or decrease the transmission power of a signal transmitted to the base station. When the SIR of a signal received from the mobile station is below a minimum value which is necessary to secure the predetermined minimum communication quality, the base station transmits TPC information instructing the mobile station to increase the transmission power to the mobile station. Conversely, when SIR of a signal received from the mobile station is greater than the minimum value, the base station transmits TPC information instructing the mobile station to decrease the transmission power to the mobile station. SIR of a reception signal is one criterion that indicates the radio communication quality. Based on the TPC information received from the base station, the mobile station controls the transmission power of a signal to the base station.
3GPP TS 25.214 V3.12.0, p21-27 (Apr. 4, 2003) shows transmit power control for transmit power of downlink signals. That is, an uplink signal transmitted to the base station from the mobile station includes TPC information to instruct the base station to increase or decrease the transmission power of signals transmitted to the mobile station, and the base station controls the transmission power of signals to the mobile station based on the TPC information received from the mobile station.
In a wireless communication system in which different frequencies are used for each base station, communication will be interrupted during a handover procedure (i.e., function that continues communication by automatically switching base stations due to movement of a mobile station) due to frequency switchover. In the handover procedure, the mobile station continues communication by automatically switching a base station as it moves.
However, in the W-CDMA system, since a base station uses a signal of the same radio frequency, it is not necessary for a mobile station to perform a frequency switchover during a handover. Hence, in the W-CDMA system, it is possible to perform a soft handover (SHO) in which a handover is possible without interrupting communication of the mobile station (SUPEKUTORAMU KAKUSAN GIJYUTSU NO SUBETE” (Matsuo Kenichi, Tokyo Denki University Press, May 30, 2002, p230-231).
In a SHO state, a mobile station connects simultaneously to a plurality of neighboring base stations using signals of the same frequency. The criterion for a mobile station to switch base stations in a SHO state is based on power of a CPICH (pilot channel) transmitted from each base station. Since all of the base stations send a CPICH at a identical power, reception power of the CPICH at the mobile station can be used as information indicating a distance between the mobile station and the respective base station.
Switching of base stations in a SHO state is carried out on the basis of the reception power of the CPICH. More specifically, switchover processing is performed based on a set referred to as an “active set”. The term “active set” refers to a set of base stations for which the CPICH reception power at the mobile station exceeds a predetermined threshold value. The mobile station communicates with base stations that are in the active set.
The reception power of a CPICH from a base station to which a mobile station is coming closer increases in accordance with the movement of the mobile station, and then the base station is added to the active set once the CPICH reception power exceeds the predetermined threshold value. The mobile station then also communicates with the base station to which it is approaching, in addition to the base stations to which it has previously connected. On the other hand, the reception power of a CPICH from a base station from which the mobile station is moving away gradually decreases in accordance with the movement of the mobile station, and then the base station is removed from the active set once the CPICH reception power drops below the predetermined threshold value. Thus, the mobile station performs switchover of base stations during a SHO on the basis of the reception power of CPICH.
In the W-CDMA system, transmit power control is performed for uplink and downlink between a mobile station and a base station even in a SHO state.
As the transmit power control for the downlink, the mobile station transmits the same TPC information to a plurality of base stations to which it has connected for communication. The TPC information transmitted by the mobile station is created for the base station that transmits a dedicated channel with SIR that is largest among SIR of dedicated channels transmitted from a plurality of base stations connecting to the mobile station in a SHO state. That is, in a SHO state, the TPC information transmitted from the mobile station is optimized for one particular base station and is not optimized for other base stations. Herein, the term “dedicated channel” refers to a communication channel that is uniquely allocated to a mobile station among communication channels between base stations and the mobile station, and the term “common channel” refers to a communication channel that is commonly allocated to a mobile station among the communication channels between base stations and mobile stations.
However, although there are relative differences in transmission power among the respective base stations, as long as each base station performs transmit power control for the downlink in the same manner based on TPC information received from a mobile station, a power drift to be described later does not occur.
With regard to transmit power control for the uplink, in general, when a mobile station receives at least an instruction to decrease the transmission power in TPC information received from a plurality of base stations to which the mobile station connected for communication, the mobile station decreases the transmission power of an uplink signal to those base stations. Hence, when a mobile station is communicating with a plurality of base stations in a SHO state, a situation can arise in which the mobile station transmits a signal that is of sufficient power to one base station, but sends a signal that is of insufficient power to another base station.
For a base station that receives a signal with insufficient power, the number of errors in reception of the signal may increase. When an error occurs in reading the TPC information transmitted by a mobile station, the downlink transmission power control at the base station malfunctions, resulting in a phenomenon, called power drift, in which the transmission power is not controlled in accordance with the TPC information that the mobile station transmitted to the base station.
When a power drift occurs, the transmission power of a downlink dedicated channel from the base station drifts. During a SHO, when the transmission power of a downlink dedicated channel from a distant base station happens to be received with large reception power at a mobile station as the result of a power drift, the mobile station commences downlink transmit power control with the distant base station.
Further, when the reception error rate of a signal from the mobile station decreased after the start of transmission power control, transmit power control is carried out to maintain the optimal reception power at the mobile station in accordance with the TPC information from the mobile station, whereby the transmission power of a downlink dedicated channel from the distant base station is maintained at a high level.
Accordingly the transmission power of the downlink dedicated channel of a distant base station remains high, providing a decrease in the link capacity of the distant base station.
Furthermore, when the mobile station moves further away from the distant base station, communication between the mobile station and the distant base station will ultimately stopped by an SHO operation to switch base stations due to a reduction of reception power of CPICH from the distant base station.
In this case, although a call is not disconnected when the transmission power of a downlink dedicated channel from the base station nearer to the mobile station is large enough to secure minimum communication quality, when the transmission power of the downlink dedicated channel of the near base station is not large enough, the call might be disconnected.
Prior art relating to transmit power control in an SHO state in the technical field of this invention will now be described. WO 99/37111 discloses technology relating a CDMA system and transmit power control to mobile stations during a soft handover. In the CDMA system, the transmission power of a mobile station is optimally controlled during a soft handover. During a SHO, a mobile station that received TPC information respectively from a plurality of base stations calculates reliability information to implement weighting that is proportional to signal power and inversely proportional to noise power on the basis of reception power (signal power). The reception power is obtained by averaging the TPC information using a predetermined time constant and the average SIR within a recent predetermined time period. The mobile station weighs the TPC information received from each base station using the reliability information, and then performs transmit power control based on the result.
According to the technology disclosed in WO 99/37111, although uplink transmit power control during a SHO is stabilized, downlink transmit power control that takes into account the movement of a mobile station with respect to a base station is not realized. Thus, the problem of the risk of a sudden call disconnection in a SHO state due to a power drift is not solved.
Japanese Patent Application Laid-Open No. 2003-163630 discloses technology relating to transmit power control of an uplink that is performed by a mobile station during a soft handover in a CDMA system. The technology is directed to correcting unsuitable transmit power control of an uplink of a mobile station caused by an error in receiving TPC information transmitted from a plurality of base stations in an SHO state. In contrast to conventional technology in which transmit power control was conducted on the basis of only a hard decision result with respect to TPC information, a mobile station that received TPC information separately from a plurality of base stations during a SHO conducts transmit power control based on the output result of a specific soft-decision algorithm and a hard decision result. For the soft decision, a method that makes a decision based on a specific threshold value, a method that disperses a decision result at a certain probability using a random number or cyclical processing or the like after making a decision based on a specific threshold value and the like are disclosed.
According to the technology disclosed in Japanese Patent Application Laid-Open No. 2003-163630, although uplink transmit power control during a SHO is stabilized, downlink transmit power control that takes into account the movement of a mobile station with respect to a base station is not realized. Thus, the problem of the risk of a sudden call disconnection in a SHO state due to a power drift is not solved.
Japanese Patent Application Laid-Open No. 2003-338786 discloses technology relating to base stations during a soft handover in the CDMA system. The technology is directed to preventing an increase in the amount of processing for transmit power control at a base station, and also directed to stably maintaining a favorable communication state regardless of whether or not a mobile station is in a handover state. SIR of a signal from a mobile station is monitored during a soft handover, and when the SIR is equal to or less than a predetermined threshold value and a possibility of the occurrence of a reception error has arisen, downlink transmit power control based on TPC information that has a low degree of reliability from the mobile station is stopped at once to thus avoid disturbing the communication state. Further, when the SIR from the mobile station exceeds a predetermined threshold value after stopping the transmission power control, transmit power control based on the TPC information is resumed.
According to the technology disclosed in Japanese Patent Application Laid-Open No. 2003-338786, the occurrence of a power drift during a SHO can be prevented. However, downlink transmit power control that takes into account the movement of a mobile station with respect to a base station is not realized. Even for downlink transmit power control based on TPC information having a high degree of reliability, the problem of the risk of a sudden call disconnection during a SHO when the distance to a mobile station is far is not solved.
Japanese Patent No. 10-112683 discloses technology relating to transmit power control during site diversity (soft/softer handover). When sufficient reception quality cannot be maintained for a signal from the mobile station to one base station among a plurality of base stations that a mobile station is connected to simultaneously during site diversity, the error rate of a control signal transmitted to the base station from the mobile station increases, a transmit power control error becomes large and interference increases. This constitutes a cause of capacity degradation in the CMDA system. The invention of Japanese Patent No. 10-112683 is directed to preventing this capacity degradation. The technology is characterized in conducting transmit power control using another separate control signal between a mobile station and a radio access network controller in addition to the normal transmit power control between a mobile station and a base station during site diversity.
According to the technology disclosed in Japanese Patent No. 10-112683, it is possible to perform highly precise transmit power control to reduce the interference amount and increase capacity in a CDMA mobile communication system. However, Japanese Patent No. 10-112683 discloses nothing at all regarding downlink transmit power control that takes into account the movement of a mobile station with respect to a base station. Even for downlink transmit power control based on TPC information having a high degree of reliability, the problem of the risk of a sudden call disconnection during a SHO when a mobile station performs a transmit power control with a base station which is far from the mobile station is not solved.